Prehistoric Sex

Not just for procreation:

[Timothy] Taylor, whose research is published by Haworth Press in the Handbook of the Evolution of Human Sexuality, says the human attitude to sex arose from the complex interaction of physical and mental development. By comparison with modern humans, who appeared about 300,000-100,000 years ago, apes have tiny male genitals, no female breasts and are hairy. But they are easily able to distinguish the sexes because males can weigh up to three times as much as females.

Humans, by contrast, are far less easy to distinguish by size. Taylor says that prominent male genitals and female breasts developed to aid recognition of the opposite sex in creatures of similar size and shape. The similarity in size, combined with the ease of face-to-face sex, allowed intercourse to become a vital part of social interaction, communication and inventiveness.

As much bonobo as chimp.